Pneumatic tire

ABSTRACT

A pneumatic tire comprises shoulder land zones and middle land zones divided by shoulder main grooves and a crown main groove. The shoulder land zone is divided by shoulder lateral grooves into shoulder blocks. The middle land zone is formed as a circumferential rib in substance. The middle land zone is provided with first and second middle outward slots which extend from the shoulder main groove toward the axially inside beyond the widthwise center line of the middle land zone, while gradually decreasing the width and inclining with respect to the tire axial direction at an angle θ1 and an angle θ2 (θ2&gt;θ1), respectively, and terminate within the middle land zone. The open ends of the first and second middle outward slots respectively face the open ends of the shoulder lateral grooves at the shoulder main groove.

BACKGROUND OF THE INVENTION

The present invention relates to a pneumatic tire, more particularly toa tread pattern capable of satisfying both of the mud performance andnoise performance.

Pneumatic tires for 4WD vehicles for use on paved or public roads aswell as off paved roads, e.g. sport-utility vehicles, pickup trucks andthe like, are required to have a good mud performance capable ofproducing a large drive power on muddy roads and a good noiseperformance capable of generating less noise during running on thesmooth and well-paved city streets and the like.

In Japanese Patent Application Publication No. 2004-058839 correspondingto US Patent Application Publication No. 2004-0020577-A1, in order tosatisfy both of mud performance and noise performance, as shown in FIG.6, the tire has shoulder land zones rs and middle land zones rm dividedby shoulder main grooves gs and crown main grooves gc; the shoulder landzones rs are divided by shoulder lateral grooves ys into rows ofshoulder blocks bs; the middle land zones rm are formed ascircumferential ribs; and the middle land zone rm is provided withinclined middle outward slots c1 extending from the shoulder main groovegs toward the axially inside, and inclined middle inward slots c2extending from the crown main groove gc toward the axially outside.Further, the shoulder lateral grooves ys include first shoulder lateralgrooves ys1 and second shoulder lateral grooves ys2 having differentgroove widths. In the vicinity of the tread edge to which contributes tothe mud performance, in order to improve the mud performance, the groovewidth WLo of the first shoulder lateral groove ys1 is set to be largerthan the groove width WSo of the second shoulder lateral groove ys2. Onthe other hand, in the vicinity of the connections between the lateralgrooves ys1 and ys2 and the crown main groove gc which contributes tothe noise performance, the groove width WLi of the first shoulderlateral grooves ys1 is set to be closer to the groove width WSi of thesecond shoulder lateral grooves ys2 in order to decrease the groovevolume and thereby improve the noise performance. Therefore, incomparison with the conventional tire, the total groove volume of theshoulder lateral grooves is decreased and thereby the noise performancecan be improved, and at the same time, it is possible to maintain themud performance nearly equal to that of the conventional tire. Thus,this prior art is directed to the improvement in the shoulder landzones, therefore, there is room for improvement.

SUMMARY OF THE INVENTION

It is therefore, an object of the present invention to provide apneumatic tire which can be further improved in the mud performance andnoise performance, by improving the configurations of slots disposed inmiddle land zones and the arrangement of the slots in relation toshoulder lateral grooves disposed in shoulder land zones.

According to the present invention, a pneumatic tire comprises a treadportion provided with circumferentially extending main grooves, wherein

the main grooves are

a pair of shoulder main grooves disposed one on each side of the tireequator to define a shoulder land zone axially outside each saidshoulder main groove, and

one or two crown main grooves disposed between the shoulder main groovesto define a middle land zone between each of the shoulder main groovesand the adjacent crown main groove,

each of the shoulder land zones is provided with shoulder lateralgrooves extending axially outwardly from the shoulder main groove to theadjacent tread edge so that the shoulder land zone is divided intocircumferentially arranged shoulder blocks in a row,

each of the middle land zones is provided with first middle outwardslots and second middle outward slots which are arranged alternately inthe tire circumferential direction,

the first middle outward slots extend from the shoulder main groovetoward the axially inside beyond the widthwise center line of the middleland zone, while gradually decreasing the width and inclining withrespect to the tire axial direction at an angle θ1, and terminate withinthe middle land zone,

the second middle outward slots extend from the shoulder main groovetoward the axially inside beyond the widthwise center line of the middleland zone, while gradually decreasing the width and inclining withrespect to the tire axial direction at an angle θ2 more than the angleθ1 toward the same direction as the first middle outward slots, andterminate within the middle land zone,

each of the middle land zones is not provided with any lateral grooveextending across the entire width of the middle land zone and having agroove width of not less than 2 mm along its length so that the middleland zone is formed as a circumferential rib in substance, and

the open ends of the first and second middle outward slots at theshoulder main groove respectively face the open ends of the shoulderlateral grooves at the shoulder main groove.

The pneumatic tire according to the present invention may be furtherprovided with the following optional features:

each of the middle land zones is further provided with middle inwardslots, the middle inward slots extend from the crown main groove towardthe axially outside beyond the widthwise center line of the middle landzone and terminate within the middle land zone;

the shoulder lateral grooves are wide shoulder lateral grooves andnarrow shoulder lateral grooves which are arranged alternately in thetire circumferential direction, and the wide shoulder lateral groove hasan average groove width more than that of the narrow shoulder lateralgroove;

the open ends of the first middle outward slots respectively face theopen ends of the wide shoulder lateral grooves; and

the axially inner ends of the first middle outward slots oralternatively the axially inner ends of the second middle outward slotsare connected to the crown main groove through sipes having a groovewidth of less than 2 mm.

In this application including specification and claims, variousdimensions, positions and the like of the tire refer to those under anormally inflated unloaded condition of the tire unless otherwise noted.

The normally inflated unloaded condition is such that the tire ismounted on a standard wheel rim and inflate to a standard pressure butloaded with no tire load.

The undermentioned normally inflated loaded condition is such that thetire is mounted on the standard wheel rim and inflated to the standardpressure and loaded with the standard tire load.

The standard wheel rim is a wheel rim officially approved or recommendedfor the tire by standards organizations, i.e. JATMA (Japan and Asia),T&RA (North America), ETRTO (Europe), TRAA (Australia), STRO(Scandinavia), ALAPA (Latin America), ITTAC (India) and the like whichare effective in the area where the tire is manufactured, sold or used.The standard pressure and the standard tire load are the maximum airpressure and the maximum tire load for the tire specified by the sameorganization in the Air-pressure/Maximum-load Table or similar list. Forexample, the standard wheel rim is the “standard rim” specified inJATMA, the “Measuring Rim” in ETRTO, the “Design Rim” in TRA or thelike. The standard pressure is the “maximum air pressure” in JATMA, the“Inflation Pressure” in ETRTO, the maximum pressure given in the “TireLoad Limits at Various Cold Inflation Pressures” table in TRA or thelike. The standard load is the “maximum load capacity” in JATMA, the“Load Capacity” in ETRTO, the maximum value given in the above-mentionedtable in TRA or the like. In case of passenger car tires, however, thestandard pressure and standard tire load are uniformly defined by 180kPa and 88% of the maximum tire load, respectively.

The tread edges are the axial outermost edges of the ground contactingpatch (camber angle=0) in the normally inflated loaded condition.

The term “groove” is meant for a groove having a groove width of notless than 2 mm, and the term “sipe” is used to refer to a very narrowgroove or cut having a groove width of less than 2 mm. A land zonecrossed by lateral grooves is called a block row or a row of blocks. Aland zone crossed by sipes only is called a circumferential rib 21.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a developed partial view of the tread portion of a pneumatictire as an embodiment of the present invention.

FIG. 2 is an enlarged partial view a middle land zone thereof.

FIG. 3 is an enlarged partial view a shoulder land zone thereof.

FIG. 4 is a plan view of a zigzag groove in a trapezoidal waveform.

FIGS. 5(A), 5(B), 5(C) and 5(D) are developed partial views of the treadportions of pneumatic tires as comparative examples.

FIG. 6 is a developed partial view of the tread portion of the prior arttire.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described in detail inconjunction with accompanying drawings.

The pneumatic tire according to the present invention comprise, asusual, a tread portion 2, a pair of axially spaced bead portions eachwith a bead core therein, a pair of sidewall portions extending betweenthe tread edges and the bead portions, a carcass extending between thebead portions, and a tread reinforcing belt disposed radially outsidethe carcass in the tread portion.

The tread portion 2 is provided with circumferentially extending maingrooves 3 and 4. The main grooves 3 and 4 are a pair of shoulder maingrooves 3 disposed one on each side of the tire equator and one or twocrown main grooves 4 disposed between the shoulder main grooves 3.Thereby, a shoulder land zone 5 is defined on the axially outside ofeach of the shoulder main grooves 3, and a middle land zone 6 is definedbetween each of the shoulder main grooves 3 and the adjacent crown maingroove 4. Further, in the case of the two crown main grooves 4, a crownland zone 7 is defined between the crown main grooves 4.

Each of the shoulder land zones 5 is provided with shoulder lateralgrooves 8 having a groove width of not less than 2 mm and extendingaxially outwardly from the shoulder main groove 3 beyond the tread edgeTe so that the shoulder land zone 5 is divided into circumferentiallyarranged shoulder blocks 9 in a row 20.

Each of the middle land zones 6 and crown land zone 7 is not crossed byany lateral groove having a groove width of not less than 2 mm, and eachzone is formed as a circumferential rib 21 continuous in the tirecircumferential direction in substance.

The shoulder main grooves 3 and crown main groove 4 each have a groovewidth of not less than 7.0 mm. Each may be formed as a straight grooveor a zigzag groove.

In this embodiment, each of the shoulder main grooves 3 and crown maingrooves 4 is a zigzag groove having a trapezoidal waveform made up of afirst oblique segment K1, a first straight segment T1, a second obliquesegment K2 and a second straight segment T2 which are, as a unit P,repeated in this order in a tire circumferential direction.

As shown in FIG. 4, the first straight segments T1 and the secondstraight segments T2 extend straight in parallel with the tirecircumferential direction and are staggered so that the first straightsegments T1 are on the tire equator side of the second straight segmentsT2.

The first oblique segments K1 extends between the straight segments T1and T2 while inclining with respect to the tire circumferentialdirection.

The second oblique segments K2 extends between the straight segments T1and T2 while inclining with respect to the tire circumferentialdirection reversely to the first oblique segments K1.

Preferably, the oblique segments K1 and K2 are inclined at angles in arange of from 30 to 60 degrees with respect to the circumferentialdirection.

In the case of the main groove formed in a trapezoidal waveform, the mudcompressed in the oblique segments K1 and K2 produces a shear force ortraction force and thereby the mud performance can be improved.

The groove depth of the shoulder main groove 3, the groove depth of thecrown main groove 4, and the groove depth of the shoulder lateral groove8 are preferably set in a range of from 9.0 to 12.0 mm. It is howeveralso possible to use conventionally adopted values for such grooves.

The middle land zone 6 are each provided with first middle outward slots11 and second middle outward slots 12 which are, as shown in FIG. 2,arranged alternately in the tire circumferential direction.

The first middle outward slot 11 extends from the shoulder main groove 3toward the axially inside beyond the midpoint 6 c of the width of themiddle land zone 6 and terminates within the middle land zone 6.

The width of the first middle outward slot 11 is gradually decreasedfrom the shoulder main groove 3 to the axially inner end 11 e thereof.

The widthwise center line of the first middle outward slot 11 isinclined with respect to the tire axial direction at a non-zero angleθ1.

The second middle outward slot 12 from the shoulder main groove 3 towardthe axially inside beyond the midpoint 6C of the width of the middleland zone 6 and terminates within the middle land zone 6.

The width of the second middle outward slot 12 is gradually decreasedfrom the shoulder main groove 3 to the axially inner end 12 e thereof.

The widthwise center line of the second middle outward slot 12 isinclined with respect to the tire axial direction at an angle θ2 morethan the angle θ1.

In the first and second middle outward slots 11 and 12 in thisembodiment, the axially inner ends 11 e of the slots 11 and the axiallyinner ends 12 e of the slots 12 are located on a circumferential line xnamely, located at the same axial position on each side of the tireequator.

The angle θ1 is constant namely, the slot 11 extends straight.

The angle θ2 is constant namely, the slot 12 extends straight.

The angle θ1 is in a range of from 5 to 20 degrees.

The difference (θ2−θ1) of the angle θ2 from the angle θ1 is in a rangeof from 10 to 35 degrees.

In connection with the rate of gradual decrease in the groove width ofeach slot 11, 12, it is preferable that the taper angle α between bothsidewalls of the slot is in a range of from 5 to 15 degrees.

The open ends 11 a and 12 a of the first and second middle outward slots11 and 12 at the shoulder main groove 3 respectively face the open ends8 a of the shoulder lateral groove 8 at the shoulder main groove 3.

More specifically, as shown in FIG. 4, when the sidewalls of thefirst/second middle outward slot 11/12 are extended to the axially outersidewall 3 s of the shoulder main groove 3, a region 11Y1/12Y1 betweentheir intersections (shaded area) overlap one of the open ends 8 apartially, preferably wholly. The open ends 11 a and 12 a of the firstand second middle outward slots 11 and 12 are each located within one ofthe first straight segments T1 of the shoulder main groove 3.

The open ends 8 a of the shoulder lateral grooves 8 are respectivelylocated in the first oblique segments K1 of the shoulder main groove 3or alternatively the second oblique segments K2.

As described above, since the first and second middle outward slots 11and 12 are terminated within the middle land zone 6, during running onrough terrain, the mud in the slots 11 and 12 is prevented from beingdischarged toward the tire equator side where the ground pressure ishigher.

Further, since the first and second middle outward slots 11 and 12 aregradually decreased in the width toward the axially inside and inclinedat the angles θ1 and θ2 with respect to the tire axial direction, duringthe tire is rotated, the mud in the slots 11 and 12 is forced by theground toward the axially outward direction in which the groove widthincreases so that the mud can be easily ejected.

Furthermore, the axially outer open ends 11 a and 12 a of the first andsecond middle outward slots 11 and 12 respectively face the axiallyinner open ends 8 a of the shoulder lateral grooves 8, therefore, themud in the middle outward slots 11 and 12 can be discharged to theshoulder main groove 3 as well as the shoulder lateral grooves 8.

By these synergistic effects, the mud ejecting performance iseffectively improved, and as a result, the road surface from which mudis removed and the middle land zone 6 which is formed as acircumferential rib 21 having a higher rigidity can well contact witheach other.

As a result, the adhesion/shear force between the ground contactingsurface and the mud is increased, and the traction force is increased toimprove the mud performance.

Since the widths of the first and second middle outward slots 11 and 12are gradually varied, the mud ejecting performance can be increasedwithin a limited groove volume.

Since the first and second middle outward slots 11 and 12 are inclinedto the same direction, they do not interfere with each other todeteriorate the mud ejecting performance.

Since the first and second middle outward slots 11 and 12 have differentinclination angles θ1 and θ2, the noise during running on smooth andwell-paved roads is dispersed and, under diversified conditions, mudejecting can be secured.

If the angle θ1 becomes less than 5 degrees and/or the taper angle αbecomes less than 5 degrees, then the effect to improve the mud ejectingperformance is decreased. If the angle θ1 exceeds 15 degrees, then thetraction force decreases. If the taper angle α exceeds 15 degrees, sincethe adhesion/share adhesion/shear force owing to the mud in the slotsdecreases, the traction force decreases. If the angle difference (θ2−θ1)exceeds 35 degrees, as the inclination of the second middle outward slot12 is increased, the traction force decreases. If the difference (θ2−θ1)is less than 10 degrees, the effect to disperse the noise is decreased.

The axially inner ends of the first middle outward slots 11 oralternatively of the second middle outward slots 12, in this embodimentthe axially inner ends 12 e of the second middle outward slot 12, areconnected to the crown main groove 4 through connecting sipes 14 havinga groove width of less than 2 mm as shown in FIG. 2. Preferably, theconnecting sipe 14 extends straight in substantially parallel with thewidthwise centerline of the slot.

The connecting sipes 14 may be closed when in the ground contactingpatch, therefore the middle land zone 6 may be provided with a rigiditysimilar to that of a circumferential rib 21. In contrast, when outsidethe ground contacting patch, the connecting sipes 14 can provideflexibility for the middle land zone 6, and as a result, the mudejecting performance can be improved.

The middle land zone 6 is further provided with middle inward slots 13.The middle inward slot 13 extends from the crown main groove 4 towardthe axially outside beyond the midpoint 6 c of the width of the middleland zone 6 and terminates within the middle inward slot 13.

The middle inward slots 13 are inclined in the same direction as thefirst and second middle outward slots 11 and 12 at an angle θ3 withrespect to the axial direction.

In this example, the angle θ3 is not less than the angle θ1 and not morethan the angle θ2.

Thus, the number of the slots formed in the middle land zone 6 becomemore than the number of the shoulder lateral grooves 8, and it becomespossible to improve the noise performance by somewhat decreasing thewidths of the slots 11, 12 and 13, while achieving a good mudperformance.

It is preferable for the mud ejecting performance that the width of themiddle inward slot 13 is gradually decreased toward the axially outside.

The number of the middle inward slots 13, the number of the middleoutward slots 11, and the number of the middle outward slots 12 per onemiddle land zone 6 are the same.

The shoulder lateral grooves 8 in this embodiment are, as shown in FIG.3, wide shoulder lateral grooves 8A and narrow shoulder lateral grooves8B which are arranged alternately in the tire circumferential direction.

The wide shoulder lateral groove 8A has an average groove width largerthan that of the narrow shoulder lateral groove 8B.

Here, the average groove width is defined as the quotient of the area Sof the shoulder lateral groove 8 at the groove top measured between theshoulder main groove 3 and the tread edge Te which is divided by theaxial length of the shoulder lateral groove 8 measured between theshoulder main groove 3 and the tread edge Te.

In this embodiment, at any axial position, the axial groove width of thewide shoulder lateral groove 8A is more than the axial groove width ofthe narrow shoulder lateral groove 8B.

The open ends 11 a of the first middle outward slots 11 respectivelyface the open ends 8Aa of the wide shoulder lateral grooves 8A, and theopen ends 12 a of the second middle outward slots 12 respectively facethe open ends 8Ba of the narrow shoulder lateral grooves 8B as explainedabove.

As explained above, since the shoulder lateral grooves 8 are thealternately arranged wide and narrow shoulder lateral grooves 8A and 8Bhaving the difference average groove widths, synchronization of thepitch noise can be avoided, and the frequency spectrum of the tire noisecan be dispersed. Thus, the noise performance can be further improved.

Comparison Tests

Based on the tread pattern shown in FIG. 1, pneumatic tires of size285/60R18 (rim size 18×8 J) for 4WD having specifications show in Table1 were prepared and tested for the noise performance and mudperformance.

All of the tires had the same specifications except for those shown inTable 1.

(1) Noise Performance Test

A four-wheel-drive car (4700 cc land cruiser) provided on the fourwheels with test tires (tire pressure 230 kPa) was coasted at a speed of70 km/h on a smooth and well-paved asphalt road in a straight testcourse, and the noise sound was evaluated by the test driver into 10ranks, wherein the higher rank number is better and the comparativeexample 1 was rank 6. The results are shown in Table 1,

(2) Mud Performance Test

On a tire test course covered with mud of about 20 cm depth, in thestopped state of the above-mentioned test car, the engine was revved upto 3000 rpm and suddenly the clutch was engaged to start the test carwith the first gear. And the initial stage acceleration (initial stageG) and longitudinal grip at the time were evaluated by the test driverinto ten ranks, wherein the higher rank number is better and thecomparative example 1 was rank 6. The results are shown in Table 1.

As apparent from the test results, embodiment tires Ex.1-Ex.4 weresuperior in both of the mud performance and noise performance tocomparative tires Ref.1-Ref.5.

TABLE 1 Tire Ref. 1 Ex. 1 Ref. 2 Ref. 3 Ref. 4 Ref. 5 Ex. 2 Ex. 3 Ex. 4Tread pattern (FIG. No.) 6 1 5(A) 5(B) 5(C) 5(D) 1 1 1 First middleoutward slots present present present present present present presentpresent present angle θ1 (deg.) 20  20 20 −6 20 20 20 20 20 taper angleα (deg.) 0 11.5 11.5 11.5 0 11.5 11.5 5 15 Do open ends face those ofshoulder lateral grooves? no yes no yes yes yes yes yes yes Secondmiddle outward slots absent present present present present presentpresent present present angle θ2 (deg.) — 34 34 34 34 20 34 34 34 taperangle α (deg.) — 8 8 8 0 8 8 5 15 inclining direction (*1) — same samereverse same same same same same Connecting sipe — present presentpresent present present absent present present Do open ends face thoseof shoulder lateral grooves? — yes no yes yes yes yes yes yes Noiseperformance 6 6 6 6 6 5.5 6 6 6 Mud performance initial stage G 6 7.56.8 6.5 6.7 6.5 7.4 7.2 7.2 longitudinal grip 6 8 6.9 6.9 7.1 7.1 7.97.6 7.4 (*1) same: the inclining direction of the first middle outwardslots is the same as that of the second middle outward slots. reverse:the inclining direction of the first middle outward slots is reverse tothat of the second middle outward slots.

REFERENCE SIGNS LIST

-   2 tread portion-   3 shoulder main groove-   4 crown main groove-   5 shoulder land zone-   6 middle land zone-   6C widthwise center line of middle land zone-   8 shoulder lateral groove-   8 a open end of shoulder lateral groove-   8A wide shoulder lateral groove-   8Aa open end of wide shoulder lateral groove-   8B narrow shoulder lateral groove-   8Ba open end of narrow shoulder lateral groove-   9 shoulder block-   11 first middle outward slot-   11 a open end of first middle outward slot-   13 second middle outward slot-   12 a open end of second middle outward slot-   13 middle inward slot-   14 connecting sipe-   20 shoulder blocks' row-   21 circumferential rib-   Te tread edge

The invention claimed is:
 1. A pneumatic tire comprising a tread portionprovided with circumferentially extending main grooves, wherein the maingrooves are a pair of shoulder main grooves disposed one on each side ofthe tire equator to define a shoulder land zone axially outside eachsaid shoulder main groove, and two crown main grooves disposed betweenthe shoulder main grooves to define a middle land zone between each saidshoulder main groove and the adjacent crown main groove, wherein each ofthe shoulder main grooves and the crown main grooves is a zigzag groovehaving a trapezoidal waveform, the configurations of the two crown maingrooves being rotationally symmetrical about a point on the tireequator, each of the shoulder land zones is provided with shoulderlateral grooves extending axially outwardly from the shoulder maingroove to the adjacent tread edge so that the shoulder land zone isdivided into circumferentially arranged shoulder blocks in a row, eachof the middle land zones is provided with first middle outward slots andsecond middle outward slots which are arranged alternately in the tirecircumferential direction, the first middle outward slots extend fromthe shoulder main groove toward the axially inside beyond the widthwisecenter line of the middle land zone, while gradually decreasing thewidth with the widthwise center line inclining with respect to the tireaxial direction at a substantially constant angle θ1 of 5 to 20 degrees,and terminate within the middle land zone, the second middle outwardslots extend from the shoulder main groove toward the axially insidebeyond the widthwise center line of the middle land zone, whilegradually decreasing the width with the widthwise center line incliningwith respect to the tire axial direction at a substantially constantangle θ2 which is greater than the angle θ1 toward the same direction asthe first middle outward slots, and terminate within the middle landzone, wherein the difference (θ2−θ1) between the angle θ1 and the angleθ2 is 10 to 35 degrees, each of the middle land zones is not providedwith any lateral groove extending across the entire width of the middleland zone and having a groove width of not less than 2 mm along itslength so that the middle land zone is formed as a circumferential ribin substance, and the open ends of the first and second middle outwardslots at the shoulder main groove respectively face the open ends of theshoulder lateral grooves at the shoulder main groove.
 2. The pneumatictire according to claim 1, wherein each of the middle land zones isfurther provided with middle inward slots, and the middle inward slotsextend from the crown main groove toward the axially outside beyond thewidthwise center line of the middle land zone and terminate within themiddle land zone.
 3. The pneumatic tire according to claim 1 or 2,wherein the shoulder lateral grooves are wide shoulder lateral groovesand narrow shoulder lateral grooves which are arranged alternately inthe tire circumferential direction, and the wide shoulder lateral groovehas an average groove width more than that of the narrow shoulderlateral groove.
 4. The pneumatic tire according to claim 3, wherein theopen ends of the first middle outward slots respectively face the openends of the wide shoulder lateral grooves.
 5. The pneumatic tireaccording to claim 1, wherein the axially inner ends of the first middleoutward slots or alternatively the axially inner ends of the secondmiddle outward slots are connected to the crown main groove throughsipes having a groove width of less than 2 mm.
 6. The pneumatic tireaccording to claim 1, wherein each of the middle land zones is providedwith middle inward slots, and the middle inward slots extend from thecrown main groove toward the axially outside beyond the widthwise centerline of the middle land zone, while inclining in the same direction asthe first and second middle outward slots at a substantially constantangle θ3 with respect to the axial direction, angle θ3 being not lessthan the angle θ1 and not more than the angle θ2, and terminate withinthe middle land zone.
 7. The pneumatic tire according to claim 1,wherein each of the main grooves is made up of axially inner straightsegments, axially outer straight segments, and oblique segmentsextending therebetween, and the first middle outward slots and thesecond middle outward slots are alternatively extended from the axiallyinner straight segments.
 8. The pneumatic tire according to claim 1,wherein the axially inner ends of the first middle outward slots and thesecond middle outward slots are positioned at the substantially sameaxial position.
 9. A pneumatic tire comprising a tread portion providedwith circumferentially extending main grooves, wherein the main groovesare a pair of shoulder main grooves disposed one on each side of thetire equator to define a shoulder land zone axially outside each saidshoulder main groove, and one crown main groove disposed between theshoulder main grooves to define a middle land zone between each saidshoulder main groove and the crown main groove, wherein each of theshoulder main grooves and the crown main grooves is a zigzag groovehaving a trapezoidal waveform, each of the shoulder land zones isprovided with shoulder lateral grooves extending axially outwardly fromthe shoulder main groove to the adjacent tread edge so that the shoulderland zone is divided into circumferentially arranged shoulder blocks ina row, each of the middle land zones is provided with first middleoutward slots and second middle outward slots which are arrangedalternately in the tire circumferential direction, the first middleoutward slots extend from the shoulder main groove toward the axiallyinside beyond the widthwise center line of the middle land zone, whilegradually decreasing the width with the widthwise center line incliningwith respect to the tire axial direction at a substantially constantangle θ1 of 5 to 20 degrees, and terminate within the middle land zone,the second middle outward slots extend from the shoulder main groovetoward the axially inside beyond the widthwise center line of the middleland zone, while gradually decreasing the width with the widthwisecenter line inclining with respect to the tire axial direction at asubstantially constant angle θ2 which is greater than the angle θ1toward the same direction as the first middle outward slots, andterminate within the middle land zone, wherein the difference (θ2−θ1)between the angle θ1 and the angle θ2 is 10 to 35 degrees, each of themiddle land zones is not provided with any lateral groove extendingacross the entire width of the middle land zone and having a groovewidth of not less than 2 mm along its length so that the middle landzone is formed as a circumferential rib in substance, and the open endsof the first and second middle outward slots at the shoulder main grooverespectively face the open ends of the shoulder lateral grooves at theshoulder main groove.
 10. The pneumatic tire according to claim 9,wherein each of the middle land zones is further provided with middleinward slots, and the middle inward slots extend from the crown maingroove toward the axially outside beyond the widthwise center line ofthe middle land zone and terminate within the middle land zone.
 11. Thepneumatic tire according to claim 9, wherein the shoulder lateralgrooves are wide shoulder lateral grooves and narrow shoulder lateralgrooves which are arranged alternately in the tire circumferentialdirection, and the wide shoulder lateral groove has an average groovewidth more than that of the narrow shoulder lateral groove.
 12. Thepneumatic tire according to claim 11, wherein the open ends of the firstmiddle outward slots respectively face the open ends of the wideshoulder lateral grooves.
 13. The pneumatic tire according to claim 9,wherein the axially inner ends of the first middle outward slots oralternatively the axially inner ends of the second middle outward slotsare connected to the crown main groove through sipes having a groovewidth of less than 2 mm.
 14. The pneumatic tire according to claim 9,wherein each of the middle land zones is provided with middle inwardslots, and the middle inward slots extend from the crown main groovetoward the axially outside beyond the widthwise center line of themiddle land zone, while inclining in the same direction as the first andsecond middle outward slots at a substantially constant angle θ3 withrespect to the axial direction, angle θ3 being not less than the angleθ1 and not more than the angle θ2, and terminates within the middle landzone.
 15. The pneumatic tire according to claim 9, wherein each of themain grooves is made up of axially inner straight segments, axiallyouter straight segments, and oblique segments extending therebetween,and the first second middle outward slots and the second middle outwardslots are alternately extended from the axially inner straight segments.16. The pneumatic tire according to claim 9, wherein the axially innerends of the first middle outward slots and the second middle outwardslots are positioned at the substantially same axial position.